When I want a strap, I use a Blackrapid shoulder strap. I use that even with a 600/4 II (the left-handed strap, in that case) without issue. Similarly for handholding, I use a hand strap on the camera that means I can support the weight without relying on my fingers gripping it. Everyone is different, but with a neck strap and without the hand strap, I know I'd feel it after a day of shooting. But with the BR strap and E-1 hand strap, I can shoot with a 1-series/R3 body and a heavy lens all day without issues.
I use Peak Design Slide Lites and, throughout the day, I change between crossbody and neck as I see fit but, when I'm shooting more often, I shorten the length of the strap and go with a neck arrangement. Depending on that, I may feel discomfort on my neck and/or my forearms. The fact that, currently, my heaviest lens is the RF 70-200mm f/2.8 does help a lot, and I can shoot longer with it but, when I was using the EF 70-200 f/2.8, after say, one and a half hour of usage, I would definitely get tired. I like to bring the camera close to me when I'm actively shooting.
Considering the 28-70 and the 24-105 f/2.8 weight pretty much the same as an EF 70-200 2.8, while my hands may feel more comfortable, due to the better balance between the camera and the lens (and good ergonomics on the 28-70), I know the rest of my body will have to sustain aproximately the same weight. When I tested the 28-70, the feeling on the neck and forearms was very "familiar"
That said, I will probably buy it anyway, someday. That lens is just too tempting
